1. Field of the Invention
This invention relates to a heat-sensitive recording material which has a superior optical readability in near infrared region.
2. Prior Art
A heat-sensitive recording sheet that utilizes a heat-color-forming reaction occuring between a colorless or pale-colored chromogenic dyestuff and a phenolic material, or an organic acid is disclosed, for example, in the Japanese Patent Publication Nos. 4160/1968 and 14039/1970 and in the Japanese Laid-Open Patent Publication No. 27736/1973, and is now widely applied for practical use.
In general, a heat-sensitive recording sheet is produced by applying on a support, such as paper, film etc., the coating which is prepared by individually grinding and dispersing a colorless chromogenic dyestuff and a color-developing material into fine particles, mixing the resultant dispersion with each other and then adding thereto binder, filler, sensitizer, slipping agent and other auxiliaries. The costing, when heated, undergoes instantaneously a chemical reaction which forms a color.
These heat-sensitive recording sheets havw now been finding a wide range of applications, including medical or industrial measurement recording instruments, terminal printers of computer and information communication systems, facsimile equipments, printers of electronic calculators, automatic ticket vending machines, and so on.
Further, these heat-sensitive recording sheets omprising the combination of a leuco-dyestuff and a color-developing agent are utilized as thermosensitive labels in POS-system. However, since the color formation is in the visible region, these recording sheets cannot be adapted for reading by a semi-conductor laser in the near infrared region which is used as a bar code scanner.
Besides the heat-sensitive color-developing system in which the above colorless leuco dyestuff is used, a chelate type color-developing system under the use of metal compounds is known.
For examples, the Japanes Patent Publication No. 8787/1957 describes the combined use of iron stearate (electron acceptor) with tannic acid or gallic acid, and the Japanese Patent Publication No. 6485/1959 describes the combined use of an electron acceptor such as silver stearate, iron stearate, gold stearate, copper stearate or mercury behenate with an electron donor such as methyl gallate, ethyl gallate, propyl gallate, butyl gallate or dodecyl gallate.
Although these heat-sensitive recording sheets using a chelate type color-developing system are utilized as thermosensitive label in POS-system, etc, they have a drawback that the readability in using semiconductor of near infrared region as ar-code-scanner is insufficient.